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I'il'r inIt
A BIOLOGICAL STUDY
OP
POLYPODIOIDES
(Resui^otion Fern)
POLYPODimjI
AS
M
AIR PU^IT IN MISSISSIPPI
LOUIS J.PESSIN
DISSERTATION
Submitted to the board of UniversityStudies of the Johns Hopkins University in
conformity with the requirements for the
degree of Doctor of Philosophy,
THE JOmiS HOPKINS UlTIVERSITY
BALTIMORE, MD.
June 1923.
\n
o?i
)
TABLE 0? GOITTENTS
I. IITTRODUCTIOIT
II.GEITERAL
1.
OBSERVATIONS
2-16
1, Characteristic floras of se^^en
hatitats for
Polypodiura as exhibited by stations 1 to 7
2, Results of general
2-12
observations on the distribution
of the fern
13-14
?, Conclusions
15-16
III.SUimARY OF LITERATTrRE ON BIOLOGY OF EPIPHYTES
17-29
IV.MEASIJREMEtTT AND COMPARISON OF ENVIRONJffiNTAL CONDITIONS
3C-46
A.Climatic Conditions
1, Introduction
?0-32
2, Methods
3S-35
(a) Light
32
humidity of the air
,33
(c)
Evaporating pov/er of the air
33
(d)
Temperature of the air
(e
Temperature of the substra,tum
(b) Relative
,
34
35-46
3, Results
B.Some
33
Features of the Chemical Composition of
46-51
Polypodium. aJid Its Substratum
l.Repults of
-inaljTfJp
of the epiphyte, of the humus
and of the supporting bark
2, Acidity of the substratum
,
46-49
49-51
C, Effects of the Physical Character of the Bark on
the Distribution of the fern
51-53
D.Discussion of Influences affecting the Distribution
of the Epiphytes studied
53-59
V. PHYSIOLOGICAL STUDIES
1, Methods of
(a)
60-79
experimentation
60-65
Relative water loss from the surface of the
62-63
leaf
(b)
Water-absorbing power of the leaves of the/
fern
(c)
62-63
Influence of the loss of v/ater on the internal
structure of the leaf
64-65
2. Results
S^Physiological significance of
65-72
the
scales to the
leaf
72-76
4, Relation of v;ater- content of leaf to leaf-
movement ,
VI. HISTOLOGICAL STUDIES
VII. GENERAL SUlfiCARY
BIBLIOGRAPHY
Vita
76-80
81-82
82-84
85-87
.88
STUDY OP POLYPODIUM POLYPODIOIIiES
A BIOLOGICAL
AS
Air
AIR
I.
PLAUT
IN
MISSISSIPPI
IITTRODUCTION
The only vascular epiphyte found in the
northeastern part of Mississippi is Poly podium
polypodioides (L) A,S,Hitch« (P« xncanum Sw, )«It
occurs abundantly in the
ndt,
ire forests where it
inhabits eight or ten different species of trees and
is also commonly found growing in large clumps on the
trunks of certain planted trees in the city streets.
Though widely distributed this polypody seems to be
able to grow especially well only on certain species
of trees, On these it occurs in very large clumps;
on a number of other species it is foujad only in very
small clumps; while
on some species the polypody does
not grow at all. This restriction of the epiphyte to
certain trees led the writer to make a study of the
climatic and other conditions influencing both the
distribution of this fern on certain individual trees
and its degree of development,
II.
GENERAL OBSERVATIONS
In order to determine definitely the causes of the
distribution of P. polypodioides actually found, the writer
selected seven stations where the epiphyte occurred
and kept these under constant observation for one yeaocf
2
Station #1 was two miles south of A & M College,
Mississippi, located about 33° 30* North Latitude.
This comprised a woodlot covering several acres and
occupied
"by
"by
two distinct forest types separated
,
a small pasture, Ulmus alata.Quercus stellata
^. f alcata» S. velutina ,
Q. rv/bra . Carya ovata
.
C. alba.
and C, glabra predominated in the first part,' Of the
shrubs in this region Crataegus sp, and Rhus sp,
v.'ere
the most common ones.On the shaded forest floor v/ere
also numerous sprouts of oak and many tall herbaceous
Compo sitae, The other part of this woodlot consisted
mainly of pine woods where Pinus taeda« P« echinata
and Junipeirus virginiana predominated with scattered
individuals of Cornus florida and Celtis Mississippiensis,
Many parts of the floor of this open forest are
exposed
to the direct
rays of the sun for much of
the day and are covered by a thick layer of pine
needles.
In the hardwood section of station #1 the
epiphytic polypody grows on several species of trees.
It v/as found on the north side of Carya alba very
close to the ground, A small patch
of it was also
found growing on the ground itself close to the tree.
The epiphyte, however, occurred not only on the north
side of the tree but was found also on the northwest
and southwest sides of individual trees of the same
species, but there it is always close to the ground.
a
3.
On <^uercus falcata i^ ^^g ^^^^ ^^ ^he southwest
side about six centimeters above the soil, On
^stellata, on the contrary, the fern invariably
occurred only
hi^
in the tree, chiefly on the
larger lateral branches and on the north side
of the main trunk itself .A similar habit was observed
in one instance on Q. falcata. wl^ile on
and
few
Q.,
Q..
rubra
velutina the fern never grew more than
a
centimeters up the trunk and frequently it
was found
growing on the very base of the trunk on
the northwest side practically imbedded in a mass of
mosses and decaying organic matter.The fern was also
common on the base of Carya glabra but here it inhabited
both the northeast and southeast sides, In a very
few cases only has the polypody been found on Carya
ovata . nd there, as on
tine
red oak, it was
growing
very clc^ to the ground, and generally entangled in
a mass of mosses and lichens.
Station #2 was a hardwood area of several acres,
located on the campus of the Mississippi Agricultural
and Mechanical College, It was a woods consisting
mainly
of tall specimens of Populus deltoides.
Liquidambjar Styraciflua.Diospyros virginiana, Crataegus
sp.and a few specimens of Ulmus alataj^ln this woodlot
the ground between the trees was covered
with numerous
4.
weeds and young sprouts of various trees. Like
station #1 the floor of these woods was v/ell shaded
in the suramer.In this region the epiphytic polypody
Figa
A view of Station #2 showing the dense vegetation
covering the forest floor.
was abundant though it occurred only on the bases
of the tree trunks .On a tree of Diospyros virp:iniarat
the fern was found growing on the northwest and norteast
sides up to about thirty centimeters from the ground
and forming six individual clumps. On TJlmus alata it
occurred also on the same exposures but only five
centimeters from the groiind; on one Liquidambar Styrac if lua
the polypody inhabited the east side of thercree as high
as forty centimeters above ground, vhile on another
individual of the same
species it was found on the
5,
southeast side at ten centimeters above ground. The
occurrence of the polypody on the stem of Crataefots sp,,
the diameter of which was only eight centimeters, at
sixteen centimeters from the ground,
for the substratum of the stem
seem to furnish no adequate
v/as
indeed surprising
of so small
ajjtree
would
foothold for the epiphyte.
Fig,2
The base of the tnmk of Diospyros virpiiniarm
showing the epiphytic fern on the northwest
and northeast sides.
Station #3 was likewise situated
of the Agricultural
am
on the campus
Mechanical College, This area
comprised all of the trees growing in the immediate
vicinity of the buildings and on
campus, The predominating trees
Quercus stellata and
Q..
the streets of the
were Ulmus americana
.
Muehelenbergii , There were also
many cultivated shrubs and trees. The epiphyte inhabited
6.
nearly all the elm trees and occurred commonly on
the north and northeast sides, Frequently it was found
inhabiting
the west and south sides also. It seemed
so v/ell adapted to grow on the elm that it made very
Fig.3
Trunk of Liquidambar Styraciflua with the epiphyte
growing on the east side forty centimeters above
ground.
often an enormous mass starting at the very base of
the tree and reaching up to the lomermost branches
of the crown, On
Q.,
stellata. on the other ham d, the
fern never occurred lower than three metoj
s
above
ground and the largest mass of it was usually in the
crown on the main trunk and on the old thick branches
Two miles northeast of the campus was station #4,
It was a wooded area of several acres. The predominating
trees there were
of all
) .
Juniperus virp:iniana
(most abundant
Pinus taeda .P, echinata . and among them were
7.
dispersed a
fev/
specimens of Ulmus alata. of Celtis
Mississippiensis and along the creeks isolated
individuals of Populus deltoides., Throughout the
woods were scattered impenetrable thickets of
Prvtnus
angustifolia (?).The floor of the woods
was always shaded by the profusely branched
and was covered
junipers
with old juniper twigs and pine
needles.
Fig.4
Base of Liquidambar Styraciflua bearing the polypody
on the southeast side ten centimeters above ground.
In this region the epiphytic polypody. occurred
on practically all of the junipers. The fern seemed to
have no definite distribution on its support.lt occurred
on all sides of the juniper trunks and at times also
on the older branches. About five meters from the very
«
8.
top of the tree the fern was generally replaced
by-
lichens. Very commonly thetfem was found growing in
close association with the liverwort Frullania virginica.
with the mosses Entodon cladorrhi zans . Clasmat odon
parvulus and in a few cases also with Orthotrichum
ohiense or Leucodp .julaceus . Practically the only lichen
associated with the polypody was Parmelia centrata .
fm^
Fig.5
Crataegus sp, bearing the epiphytic polypody.
This form was abundant on the lower branches of the
tree and was almp^t imbedded in the mosses, Other
lichens occurred on thethigher parts of thetree where
neither mosses
nor ferns were present, The latter forms
were;Ramalina calicaris.Sticta sp, Physcia stellaris.
Anaptychia speciosa .
ajid
Getraria aurescens ,The only
algae found on the junipers were Tolypothrix sp.
9.
Pleurococcus sp, and Nostoc sp, and even they were not
very abundant, No epiphytic bryophytes or
ferns were
found on any species of Pinus. but lichens such as
Parmelia centrata were abundant on the trunks of many
of them.
Fig.6
Typical growth of the
fern on the american elm.
Station #5 was located
of the campus, Its
Fig. 7
A view of the juniper
woods of station#4.
four miles southwest
flora was very similar to that of
station #4 as was the distribution of the polypody
also.
Station #6 was an open field on the college
campus. There a solitary tree of quercus stellata
was located between two low ridges but fully exposed
on
10.
all sides. It measured forty eight centimeters in
diameter at one and one-half meters from the ground
and began to branch out at four meters from the ground.
This tree was selected for the ecological study of which
further mention will be made below, The epiphytic polypdy
Fig.P
„
of station 1^6,
arrangement of the
the
shows
also
This
tree for ecological study.
the
on
instruments
Q.uercus stellata
together with several species of mosses was abundant
on this tree, Wherever the fern and mosses were present
11.
together, they usually were
tangled into one huge mass.
At the base of the trunk, the mosses inhabited the north
side of the tree as well as the northeast
and northwest
sides.At this height no mosses were present on the
south side;higher up into the crown, on the contrary,
mosses alone occurred on the south side while the fern
inhabited only the northwest, north,
sjid
northeast
sides.The bryophytes associated with the fern on the
oak were Leucodon .iulaceus . Clfflnatodon parvulus rupestris.
and Frullania vir^inica.The lichens were Epra chlorina.
Parmeli a centrata,and Pertusaria wulfenii , These occurred
chiefly on the upper branches where no other green
epiphytes were present, while Parmelia perforata inhabited
both the upper branches and the parts of the trtmk
devoid of green epiphytes, At the base Leptogium sp,
was common only on the north side of the tree.
Station #7 was situated in Starkville, Mississippi
one and one-half miles from A & M College. There the
distribution of the polypody was. studied on the trees
aljng the city streets, It was found that Ulmus americana
was the only tree thickly inhabited by the
fa:'n
whidi
was always associated with various mosses. On this
elm the polypody was not limited to amy particular
exposure, nor to any particular height. On individual
trees of Que reus stellata in this region the polypody
was confined only to the upper part of the tree
inhabiting the thick lateral branches and the trunk
12,
itself. The fern was never found on the oak in this
region at an altitude less than five meters. The elm
and the oak
are the only trees on which the polypody
occurs abundantly in this region, Only one specimen
of Celtis mississippiensis was found bearing a small
clump of the polypody, This was an^usual case since
on no other specimen of Celtis of any diameter^was
the fern ever found, It is quite possible that on
Fig,9
The characteristic habit of P. r)olypodiodes
on ^uercus stellata at station #7,
this tree the fern was able to take a good hold
beacause there was much decaying vegetable matter
accumulated on a projecting piece of bark where
the epiphyte
anchored it self .It may be added that
13.
on a specimen of Carya. pecan
groviTing
(
a cultiyated variety)
close to a residence^on a side street, at
Starkville, the polypody occurred
abundantly in
numerous small clumps. This is of interest
since
'
no other Garya bore this polypody, perhaps because
no other offered as good a support to the fern as
did this pecan.
An analysis of the data at hand indicates
that where the light is weak this epiphytic fern
is restricted to no definite exposure; it occurs
Of
novi
on the north side, novf on the south-east side
and frequently
on the west side of the tree,On
one tree it is very close to the ground, on another
somewhat higher, while on some trees it gets
far
above the ground. On the other hand, where the lign
is intensQthe polypody is either
t
not present at
all or when it does occur it is found only on the
north side of the tree, It appears, therefore, that
the light plays an important role in the distribution
of the epiphyte, That the light is not jrhe only
factor affecting the occurrence of Polypodium
polypodioides
as an epiphyte is evident from the
fact that even when such trees
^inus taeda, and Carya ovata
as Pinus echlnatsy
are exposed to weak
light, they do not bear the fern.
14.
Another condition that appears to
"be
responsible
for the presence of the epiphyte on some trees and
its absence from others is the character of the substratum
This is evident from a study of a young fern plant
which the writer succeeded in removing together
with the
ba:
fern or the
k on v/hich it grew without injuring the
b^
to which it was attached, The piece
of bark bearing the young fern measured eight
centimeters
in length, four centimeters in width and one centimeter
in thickness. The fern grew on the edge of the
bark
pushing its young roots into the interlaminar spaces
so that the youngest roots branched out luxuriantly
about two centimeters
beneath the surfe ce of the bark.
Fig.lO
Upper surface of bark of
elm bearing the polypody
on its edge.
In this manner the fern roots
Pig. 11
Lower surface of bark with
the polypody showing the
root system.
were
really concealed
in the bark half way through its thickness, In this
position the young absorbing roots were covered by
a thickness of bark sufficient to serve as an excellent
protection against
aes'iccp.tion
Even when the fern
on the outside was apparently dried up, shriveled and
15.
curled up, as it always dLm- in dry weather, its roots
"beneath the "bark were perfectly turgid and showed
no apparent suffering from lack of moisture. This
olDservation suggests at once
that the character
of the "bark is as important a factor in the distribution
of the epiphyte as is the light, Even when the light
conditions are favorable, a tree possessing a homy,
a stony, or a flaky bark does not furnish
the
proper support for the polypody, since its roots
can not penetrate into the stony or horny bark,while
on a flai:y bark that sheds frequently, such as of
the pine or the sycamore, the young epiphyte
lias
hardly
time to develop strong anchoring roots when the bark
is shed carrying with it the young plant to the ground.
In most cases, however, the spore fails to germinate,
for no prothallium has ever been found on either
the pine or sycamore in the northeastern part of
Mississippi.
It seems possible that both light and the
character of the substratum might be responsible
for the diversity in the distribution of the epiphytic
fern as it appears from a field
study of the seven
stations. To determine this definitely the writer
was le
I
to
carry on
quantitative physiological
and ecological experiments in two of the observed
stations for a definite period of time. It was thought
16.
possible to establish the actual causes underlying
the influence of the climatic conditions and of
the character of the substratum on the distribution
of the epiphyte. Before presenting the data obtained
from the physiological, ecological and morphological
study of this epiphyte indigenous to the Southeastern
United States, it is perhaps well to give a brief
review of the literature on epi::hytes in geneia.1
so as to follow up the ideas on epiphytism as
conceived
by the earlier botanists and particularly
to sho^how little
is actually known about this
interesting group of plants.
The writer wishes to express his thanks
Professor C.C.Plitt for the determination of
several of the lichens and to Mr,RoS,Williams
of the New York Botanic Gardens for the
determination of some of the epiplitic mosses.
to
«
.
17,
OF LITERATURE ON BIOLOGY OP EPIPHYTES
III. SUMMARY
Practica,lly nothing was known to the early
botanists about the ecological group of plants
now known as
epiphytes, It was late in the nineteenth
ceatuty when the first papers dealing with these
air plants
question of
b^gan
to appeaJC.As early as 1881 the
the source
of the mineapl elements
necessary for the growth and development of the
epiphytes interested botanists.It was then that
Dixon (4) analyzed chemically the ash of living
and dead fronds as well as roots of Platycerium
grande P« aM come
and Asplenium nidus He likewise
analyzed the ash of the humus of the substratum on
which these epipht«<es were growing, His results
indicate that there was no similarity betv/een the
composition of the ash of the living epiphyte and
that of the substratum to which it was attached.
He concluded from this that the epiphytes obtained
most of their inorganic matter from the dust of the
air, Although these, plants generally require "quantities
of
alkalies", they frequently obtain these substances
from decaying fronds and from the humus.
The foremost student of epiphytes was Schimper."
His paper (24), the results of his travels in the
southern part of the United States and the West Indies,
caafQvts^eii
a great deal of enthusiasm over epiphytes
,
18.
among many European totanists.In his first paper
on epiphytes he classified the air plants into
groups according to their
four
anatomical structure and
their adaptation to the environment. Those epiphytes
which derive their nutrient material mainly fran the
surface of the suhstratum to which they are attached
he placed into the first group. In this group are the
orchids and the aroids which get their
mineral nutrients
trough, the velamen of the roots. The
and water
plants
of the second group are those that send roots, which
are hoth anchoring and ahsorhingjdown into the substratum,
Clusia and Antjaurium pelmatum belong to this group.
In the third group are those epiphytes which form root
mats and
which may also have both anchoring and
absorbing ro o t s Po ly po dium phyllitidis and Oncidium
altissiraum
are examples of this group. In the fourth
group belong those epiphytes which secure their
nutrient materials entirely through leaves and lose their
roots altogether, A striking example of this group
is Tillandsia u sneoides ,
Schimper's classic v;ork on epiphytes
appeared
several years later, It was then tha.t the complete
results of his study of theitropical epiphytes were
first
published (25), It is safe to say that this
paper stimulated all the work on European "epiihytes"
that was carried on during the
re
xt several years.
19.
According to Schimper,the condition
which enables
a plant to "become an epiphyte is the adaptibility
of the seeds to be transported to branches and
triinks of trees and be able to germinate there.
Only certain seeds ^for such a mode of distribution.
He classified the seeds into three categories
according to their special adaptations: Seeds which
are fleshy, seeds v/hich are very light, and seeds
which posses special mechanical devices for dissemination.
The seeds of the first category are carried to the
trees by animals; those of
tie
second category are
transported by the wind due to their lightness; the
trees by
seeds of the third category are brought to the
means of their special flying apparatus and the agency
of the wind.
Although a seed may have those special adaptations
for dissemination and be able to germinate
on the
branch or on the trunk of the tree, the young plant
may not be able to stand the new conditions and may
require new adaptations which frequently result in
special modifications of the plant body in order to
meet the new conditions. The most marked modifications
are evident among those plants which are adapted to
resist drought. Some forms such as Poly podium polyT^odloides
are especially adapted to withsatnd the strong sunlight
in Trinidad by curling up, thus reducing its transpiring
surface, Other plants produce special structures which
20,
store water in the wet season for their later use.v'hen
the supply of water is not so abundant Thi s is especially.
true for Utricularia and some of the aroids. Still
others form special tissues which are capable of absorbing
water rapidly as is evident in the orchids .In these plants
the velamen, which is so common in epiphytic orchids^ is
capable of absorbing water rapidly and storing it for
some time. Moreover, the aerial roots of the epiphytic
orchids in addition to the velamen also have considerable
chlorophyll tissue in the cortex.They are thus capable
of photosynthetic work.
Schimper further showed that
in some bromeliads
the leaves and not the roots assume the function
water
absorption. In such forms
of
as Tillandsia usneoides,
function
scales which cover the entire body of the plant
was
as absorbing organs.The structure of such scales
later
worked out in detail
by Mez (18). Schimper 's
experiments with the leaves of Caraguata lingulata,
Brocinia. and Vresia sp. prove conclusively that in these
forms the leaves are the chief organs
Of
absorption.
Concerning the distribution of epiphytic species,
Schimper regards light and humidity as the chief condition
affecting the occurrence
of the epiphytes in special
localities.He states (p.90);» Licht.f euchte Luft,
reichliche Thaubildung.hauf ige Regenguesse stellen die
21.
wessentlichen Bedingungen eines uppigen epiphytischen
Pflanzenletens dar."
Goe"bel
(7) studying some species of
Polypodium
and Asuleniiun nidus conlude« that the Polypodium
can grow
on tbs ground as well as on trees .The
Aspleniiam. however, builds a substratum about its
roots by means of the basal leaves v/hich collect
humus therby allowing the creeping stem to penetrate
and the roots to thrive within the substratum thus
formed.Later Goebel (8)observed
tlrat
when the seed
of an epiphyte lodges on the bark of a tree and
succeeds in germinating there, its main root, in the
course of
time, flattens out and numerous side roots
substratum,'
are developed which anchor the plant to its
He found that on the smooth stem of Bambusa only
crustaceous
up
lichens occur.He also found that higher
epiphytes do not inhabit the tall smooth barked
palms, while on the persisting lower portions of the
leaf bases of the sugar palm many epiphytes thrive
in the accumulated huraus.He concluded that the
character of the substratiim plays
an important role
this
in the distribution of the epiphytes •According to
investigator, the ability to be anchored to the
substratum, an abundant supplly of water,
ajrid
an
accumulation of humus within the reach of the roots
as well as protection
for the roots against dessication
mode
are necessary conditions for the epiphytic
of life.^
22,
The appearance of Schimper's and Goebel's
published works on
the epiphytes created an interest
in this group of plants among other Europeeui
botanists.Loew (15) studied the "epiphytes" inhabiting
the willows of North Germany and found that fifty three
percent of the "epiphytes" were brought to the tree by
the agency of the wind due to the lightness of their
seeds. Twenty three percent of the "epiphytes" originated
from seeds brought by animals .The remainder of the
"epiphytes" found their way to their supporting trees
by unknown means .As a result of his study of the
"epiphytes" he concludes that
those plants which assume
the epiphytic mode of life become associated with
mycorhiza on the tree which they inhabit, This is merely
£in
assumption, for he presents
no evidence
in support
of his view, He regards plants which are able to secure
substances from humus by means of mycorhiza as better
fitted to live as epiphytes than those
v/hich do not
have that ability.lt is probable that Loew's interest
in this problem was inspired by Goebel's work rather
than by Schimper's,
Loew's view on the mycorhizal association in
epiphytes was later disputed by other v/orkers. Willis
that
and Burkill (32) pointed out a mycorhizal association
is not necessary for epiphytic development. They found
"epiphytes" occurring on the pollard willows near
Cambridge, Englad, in old and fully decayed humus
23.
and no mycorliiza was found
in thelroots
of the
"epiphyte ".Hoeveler (12) claims that plants may make
use
of
humus without the aid of mycorhiza.Beyer (l)
also failed
to find mycorhiza in the "epiphytes" that
he studied, Stahl (28) studying several epiphytic ferns
including Polypodium vulgare confiimed Beyer's results^
for he fotind no mycorhiza in any of the plants studied,
Staeger (27) likewise failed to confirm Loew's view
regarding the presence
of mycorhiza in epiphytes.
Czapek (3) in his study of the physiology of epiphytic
orchids found no mycorhiza associated with the roots
of those plant S.Johnson (13), on the other hand,
in his
study of Polypodium vulgare states (p.239):
"Many of these root hairs (of the polypody) had one
or more fungus hyphae running lengthwise through them.
These hyphae could often be seen enetering at the tip
of the root ha,irs."He has not determined
,
however,
whether these hyphae formed a mycorhizal association
or not.
Beyer (l) found that the "epiphytes" "belonged to
groups of plants
whose fruits
or seeds are stone-like,
bur-like, or have ^flying, apparatus, or are very small
and light .He concludes that the wind and the tree-
inhabiting animals aid in the distribution of the
"epijihytesfiXSeisenheyner's observations (5) confirm
those of Beyer. In another paper Beyer (2) states:
"only those plants can
live as epiphytes which can
24.
survive on the meager fertility of the substratum
which
cstn
stand
the hot sun and the drying
ajid
of the
winds ." Karsten (14) found ^in Polypodium imbricatum
u
and Po si no sum scales were present v/hich functioned
as water absorbing organs as well as
for the distribution
of the water drops which settle on the stem.He assumes,
however, that the
scales are not the sole
water
absorbers, for he found long, slightly thickened root
hairs which he believed
to function
as the main organs
of absorption, Wittrock (33) found no true epiphytes
in Sweden, He suggests that in order to assume the
epiphytic mode of life the plant must be
much shade; it must be able to form roots
thrive
in a thin layer of soil
,
ajid
ab^
to withstand
v/hich could
it must be able
to withstand dessication. He agrees with Schimper, Beyer
and the others on the characeter of the seed which
enables a plant to make its start as an epiphyte.
Went (30)
made observations on epiphytes in Java and
found that the aerial roots of epiphytes vary in form
and behavior even
in the same species, Some roots
may function only for anchor age, others for absorption
only and some for both anchorage and absorption, He
distinguishes
and
tv/o
groups of epiphytes: True epiphytes
Herai epiphytes, The
true epiphytes derive their
inorganic substances from the air or from the dust
accumulated on the bark; the hemiepiphytes are similar
to true epiphytes in their first stages of development
25.
"but
later develop
absorbing roots by means of
v/hich.
they grow as terrestrial plants.He believes that the
distribution of the epiphytes depends on the presence
or absence of inorganic material «He suggests that
epiphytes have been evolved from root climbers which
at first possessed only anchoring roots,] e,ter absorbing
roots were also formed, sjid so the plant^a "hemi epiphyte";
finally, both the anchoring
and the absorbing roots
ceased to function and the plant
Nabokisch (20) found that
with the condensation of
became a"true
epiphyte".
the velamen is not concerned
of atmospheric humidity, It
serves chiefly as a good protection against dessication
and against sudden cooling of the roots, Where the mists
are very heavy the velamen may absorb the drops of water
which form on the roots, He believes that the velamen
can absorb water only in the form
of liquid,Lindsbauer
(16) found that the typical absorbing roots of the
aroids are posit;\rely
geotropic while the anchoring
roots are usually negatively geotropic, Staeger (27)
searched for any possible adaptations among the*chance
epiphytes" of Switzerland in order to determine
any
possible relation between the European "epiphytes"
and the tropical true epiphytes, He found no special
adaptations
for the epiphytic mode of life among the
European "epiphytes ".This investigator
the first time the
applied for
name of "accidental epiphytes" to
those forms studied by
Loew, Beyer, andlthe other students
26,
of the European''epiphytes'',He regards the factors
v/hich help
"both
the existence of the "epiphyte" as
external smd internal, The external factor is
the ability to grow in a shady and humid environment;
the internal, the hereditary adaptaion to withstand
dessication, These are the only adaptations possessed
"by
the "accidental epiphytes ".Czapek (s) studied the
morphology and physiology
of epiphytic orchids and
found that the roots showed positive geotropism.Some
orchids,however, possess roots
which are negatively
helio tropic, When aerial roots were grown
in the dark
numerous root hairs were developed, Furthermore, he foimd
that only liquid water is absorbed by the roots
in
appreciable amount, The velamen may condense atmospheric
moisture but the quantity of such moisture is so
small that the velamen can not be considered chiefly as
a condenser of atmospheric humidj.ty, but should be regarded
primarily as a water absorbing organ, Through a continual
absorption of small quantities of water, the roots
really perform the function of supplying the plant
with the necessary amount of v/ater.
Of the distribution of the epiphytes in Jamaica,
Shreve (26) says {p.l87)
:
"The trees of the upper
slopes have the same epiphytic flora that would be
found in the upper two- thirds of the trees of the
ravines,The trees of the ridges and peaks have only
those that are characteristic of the uppermost
third and the midheight species are restricted in
27.
these habitats to the sides of prostrate trunks
or fallen logs»"He found that the I^ymenophyllaceae
obtain most of their water through the absorbing
power of their leaves, though the roots
also function
as water absorbing organs, The drought-resisting
species are capable of absorbing
atmospheric humidity
when the leaves are placed in an environment of very
moist air especially if the surface of the leaves is
dry.Miehe (19) found that Polypodium vulgare was
confined
to the branches of the oak.He also found
that Sorbus» Amelanchier. end Sambucue occurring
as "epiphytes" were not restiricted
to any particular
side.Olsen (21) found that epiphytic bryophytes
are mo'^ commonly found on the upper side of inclined
trunks.This, he thinks, is due to the rain falling
vertically in the woods so that the upper side of
the inclined trunk gets the most moisture, The conditions
which
affect the bryophytic vegetation, according to
Olsen, are the age of the tree, position in regard
to light, position in regard to wind,position in regard
to rainfa21,the species of the tree, and the chemical
composition of the "soil, Of these, the age of the tree
is the most important one since
it affects
the
character of the bark,Young trees possessing smooth
bark offer no footing for epiphytic grov/th.When the
tree becomes older fissures appear in the bark in
)
28,
sufficient soil accumulates to allow the
v/hich
the epiphytic
bryopliytes to appear.He found
bryophytes to
"be
xerophyllous in character. They
their
possess high osmotic pressure in the cells and
leaves
in dry weather can fold and hug the stem.
He also found
that most of the epiphytic bryophytes
can endure a high degree of
In recent years
aspects
slaade."^
the physiological and ecological
of the epiphytic mode of life began to
interst botanists.Harris (10 ) after studying the
osmotic concentration of the tissue fluids of epiphytes
concludes that these plants
shov/
a decidedly
lov/er
Gonoentration than those from the terrestrial
vegetation.Harper (9) analyzed chemically the ash
suggests
of the fronds of Polypodium polypodioides and
inorganic
that this fern probably obtains some of the
material from the
bark of
the tree which it inhabits
as well as from the dust of the air. Wherry (31
applyir-g the ^indicator tests to the "soils" on which
certain epiphytic ferns
were growing found that
Polypodium Tulp:are frequently grows on calcareous
plant,'
soil while P. oolypodioides is practically an acid
Johnson (13
)
suggests that the main difference between
the "half-parasite
P. wlgare is that
",
(
the mistletoe, and
p.lil)
"
the epiphytic
the mistletoe exacts
within
its quota of salt (and of water also) from
by the
the living host, before they have been used
29.
host itself, while the air plant gets its salts from
the surface of the tree after they
have served the
function within it." As to its origin in the United
States, he suggests that it entered North America
from Eurasia Ly way of Alaska and then spread southward
and eastward
.
Van Oye (22) concludes from his
experiments and observations
on some epiphytic
<^^^
algae, lichens and mosses in Java that the lichens
favor low humidity
and intense sunlight, This is
distribution
in accord with Plitt's (23) work on the
mosses
of lichens in Jamaica.Van Oye also found that
develop only in the
s%^
,
while the alga TrentepoMia.
sp, seems to favor strong light.
30,
IVilCBASUEElCLNT,AJVTD'GOI-,TPARISOU
OP EIIVIR0M5E1TTAL
CONDITIONS,
A. Climatic Conditions
A review of the botanical literature shows
that the early students
of epiphytes studied them
priniarily to determine the means
"by
which these
plants reach the ^itr trees ¥;hich bear them.This led
to the study of dissemination of various seeds, Later
the investigation of the nutrition of the epiphytes
and of the structure of the roots of the epiphytes in
search of a possible
raycorhizal association was
begun.Certain investigators studied the true tropical
epiphytes, while others were studying the "false"
epiphytes that occurred in Europe.The results showed
that the true air plants have special peculiarities
of structure which enable them to live and to grow
without any connection with the soil. The "false"
epiphytes, on the contrary, have no such Rdar'''itions
and few reach maturity on their supporting tree, Only
those reach maturity, of course, which
afjf^ter
a time
succeed in making connection with the earth through
the tissues of its support, Finally, the physiological
and ecological phases of the problem have attracted
the attention of recent workers who have attempted
to determine
the osmotic concentration of the sap
of epiphytes and the conditions under which tiB
air plant grows best, It seems that the prevailing
31.
idea among the students of the epiphytes has been
that light and humidity are the main conditions
that influence the distribution of these plsuits.
However, no experimental evidence has so far been
given in support of this view.'
It is the aim of this paper to present
the
results that were obtained from a series of experiments
carried on
from July 1921 to July 1922 chiefly
on Q.uercu3 stellata which was thickly inhabited.
at certain heights and on certain exposures by the
epiphytic Polypodium polypodioides . The oak
was
located in an open pasture with no buildings orlother
trees within a radius of three hundred feet or more,The epiphytic flora and the location of this particular
tree
were described in detail above (station #6
This oak
was particularly
),
desirable for the study
of the influence of the climatic conditions on the
distribution of the epiphytic polypody, for its isolated
position exposed it
on all sides to the unmodified
natural climatic conditions.'
Prior to the work on the oak, preliminary
experiments were also madaon a specimen of Jtmiperus
virginiana
which was inhabited by the fern, On this
tree the polypody
favored no definite exposure, The
location of the juniper and its epiphytic flora are
32,
discussed in detail in the section dealing v/ith
station #4. The experiments were carried
on this
tree from July 4th to July 8th, 1921 at a height
of
12«2meters;from July 8th to July 14th, 1921, at 7.6
meters; and from July 14th to July 22nd, 1921, at
1*2 meters from the ground, At each of these altitudes
the instruments were placed on the south, northeast
and northwest sides,
He
It is
practically impossible at present time
to measure some climatic conditions with absolute
prcision, However, methods adapted to experimentation
in the field were employed which enabled the writer
to obtain data of sufficient accuracy for our purpose
to make the results significant, To measure the intensity
of the light the writer employed tie
Wynne ITiotographic
Exposure Meter which was read at each station on the
tree, The readings were compared with those obtained
in the open, When the instrument was exposed in the
open field at noon for five successive
days in July
under the same conditions, it took about 1,5 seconds
to change the
paper to the standard col or, It was
then decided to use 1,5 as the standard or maximum
light intensity, All the other readings were obtained
by placing the instrtunent
against the tree and
recording the time it took the light to change the
col r of the photographic paper, These readings were
computed as percentages of the intensity of the light
in the ppeni^
33.
The relative hiimidity of the air was measured
"by
means of the sling psychrometer, which was used
not by whirling it, as is commonly done,
"but
by taking
the instrument out of its case and hsinging it on a
nail against the tree trunk where it was fanned by
as uniform a swing of the arm as possible, This method
made it possible to obtain the percentage
of relative
humidity close to the trunk or branch on which tne
ferns live .The percentage of relative humidity was
calculated from the psychrometric tables^
The evaporating power of the air in the immediate
vicinity of the stem was measured by means of ^h^
Livingston standardized
spherical porous-cup alanometers
which were set up with the non-rain absorbing mountings
as described by Livingston and Tj^one (l7)oThe atmometers
were set up in pairs at each exposure at different
heights and were read once each
twenty four hours.
After each reading the spheres were
with 95^ alcohol to remove
brushed and washed
any possible spores
o r
dust, The evaporation was measured voliometrically in
cubic centimeters.
On ftuercus stellata the temperature of the bark
was also measured in addition to the other factors, This
was done by boring a hole in the bark large enough
to hold the bulb of a thermometer, Between readings
34.
the holes thus made
heights
on three exposures at the different
were plugged with tightly fitting corks in
order to prevent the accumulation of dust, or animal,
or vegetahle
matter which might induce decay. On the
Fig.l2
Main trunk of Q., 3tellata showing position
of instruments at 1.2 and 3 meters above
ground.
oak readings were obtained
at heights 1,2 meters, 3 meters
9.1. meters, and 13.7 meters. At each altitude observations
were made on the south, northeast, and northwest sides.
With the exception of evaporation readings, which were
taken only every twenty four hours, the readings of
all the other factors
were made at nine and eleven
o'clock in the morning and two and four in the afterncon.
,
35.
Records of the climatic conditions prevailing
at A & M College during the period of experimentation
were obtained from the United States Weather Bureau
and are presented in Tahle I, It is evident from the
data that the prevailing
winds during the time
of experimentation were from the south, consequently
their drying effect was not
very great.
Results
A study of the epiphytic flora of Juniperus
virginiana in the shady woods and
of Quercus stellata
in the open, revealed the fact that on both trees the
uppermost part of the trunk bore only lichens and no
green epiphytes .The main difference between the
epiphytic floras of the two trees was noticeable at 1,2
meters from the ground where on the juniper at that
height the fern was found abundantly and was not
restricted to any particular expo sure, while on the
oak the fern was wanting altogether at that height.
Only on the upper third of the trunk of the juniper
was the polypody wanting, below that the fern was found
abundantly on the stem, On the oak, on the contrary,
the fern did not occur either at the base of the tree
or at the uppermost part of the crown where the main
axis was lost in the brsuiche3,At the other elevations,
it
occurred only on the northeast, north, and north-
west exposures.
37.
The results obtained by measuring the climatic
conditions at each altitude on both trees may shed
some light on the cause of the restricted distribution
of P.polypodioides at the different heights of the tree
and on the different exposures of the same height.
The total average evaporation per day at 12«2 meters
above ground on the juniper was 37,06 'cubic centimeters^
on the south side, 46,79 cubic centimeters on the
northeast exposure, and 37,06 cubic centimeters on the
northv»'est side of the tree, At 7,5
meters the evaporation
per day was much lower being only 14,47 cubic centimeters
on the south side, 15, 46
on the northeast side, and
14,83 cubic centimeters on the northwest side. At
1#2 meters from the ground, however, the average daily
evaporation was a trifle lower, being only 12.64 cubic
centimeters on the south side, 13, IV on the northeast
side, and 14,13 cubic centimeters on the northwest side.
The light intensity at each of the selected elevations
on the juniper was practically the same and there was
hardly any indication of its possible influence on the
distribution of the epiphyte, Unquestionably, the intensity
of the light exerts an influence on the evaporating power
of the air and in that way only does it affect the
distribution of the epiphyte.
On ftuercus stellata. where over eleven hundred
readings were recorded, the evidence
supports
the
38.
Table
II Averaf^e
Elevation Exposure
^^3,-,^^^^^^^^^^
I tijstratTim
B
^^
Querj>tis
stellata
Temperature
_
JDES
FOLD OUT
d6
El/APORATion AT D Iff EKEHT ALTITUDES
0^
^
oia
Quercus sidlata
Jun/pcrus yirfinianaB
5outw
riortKe,ast
tt.
/
u
IJL
U\m
3m
^0.\y^
iZy^
n*H
7.i*KM
/x.am
A3
A.Total average evaporation at four aliierent heic^ts on
trunk.
Q.uercus stellata on three different sides of the
5*1 6!
average eva-ooration at three different heights on
B Tot-1
trunk.
'junijyrus virfdnJPJia on three different sides of the
«',
:1a:
I
FOLD OUT
40.
30
10
h;^^::^
STAT
t,xi-ov.iiig
u rPQ-r
R
Q
tixe
relation
of the epiphytes.
Ox
yVci^oration tdthe
39.
Table III
Station
General Averages of Evaporatton, Light ,Hunidlty, Air Temperature
and Substratum Temperature on Juniperus vlrF:lniana and
?uercus stellata arranged according to the rate of
evaporation.
Location
Evaporation
in c.c.in
Humidity
%
Sub. Ej^iphytes
Air
Light
Intensity Temp. Temp.
^4 hrs.
12.64
1.2 meters
from ground
south side
J. virginiana
r^
24.00 C
62.94
1.
Fnaiania
virginica
2.Sntodon
cladorrhizans
S.Clasnotodon
parvulus
4. Leucodon
.iulaceus
S. Polypodium
polvTiodioides
B
Same as A
northeast
side
13.17
63.31
24.00 C
Same as for
station A,
Same a^
station A
northwest
side
14.13
62.10
23.9 C
Same as for
Station A
14.47
7.6 meters
above ground
south side
J. virginiana
2.6
l i'rullania
virginica
5ntodon
2.
cladorrhizans
3.Clasmotodon
parvulus
4. Leucodon
Jul ace us
S. Orthotrichun
o hlense
S. Polypodium
polypodioides
E
Same as D
northwest
side
14.83
2.8
Same as for
station D
P.
Same as D
northeast
side
15.46
5.2
Same as for
station D
40.
Table III
Station
G.
Continued
Location
Evaporation Humidity Air
Subst. Light
Temp .Temp.
9.1 meters 24.66
from ground
59.20
24.4 G 22 C
Epiphytes
6.92 l.Prullania
Northv/est
virfTJnica
2. Leucodon
side
Q. stellata
S. Polrrpodi-um
julaceus
polyp odioides
H.
3 meters
26.41
from ground
northwest
58.05
25.4 G 24.1 C 12.4 l. Leucodon
.lulaceus
2. Pol:,T3odium
Q. stellata
3 meters
polypodioides
27.49
59.32
24.6 G
25.4
from ground
northeast
9,» stellata
12.1 l. Leptobium
tremeloides
2» I'armelia
*
ce^trata
3. Clasinatodon
parvulus
mpestris
4.Lencodon
.iulaceus
S. Polypodium
polr/podioides
J.
9.1 meters 27.88
from ground
northeast
Q. stellata
59.65
25 C
22.7 C
6.6
l. Clasmatodon
parvulus
mpestris
2. Leucodon
Julaceus
g. Polypodium
polypodioides
K,
3 meters
29.75
from ground
South side
Q.stellata
58.96
25.7 C 24.8 C 12.9
l. Parmelia
centrat a
2 »Zanthoria
lychnea
S.Leucodon
.iulaceus
a
41.
continued
Talble III
Station Location
Evaporation Hunidity Air Temp, Subst. Light
Epiphytes
Temp.
9.1 meters 30,01
from groimd
58.95
25.1 C
24 C
11.42
south side
O.stellata
M.
l.Parmelia
cent rat
2,Leucodon
.julaceus
1.2 meters
33.96
from ground
northwest
side Q. stellata
58.11
25.4 C
20 C
14.33
l. Le-ptogium
treraeloide;
2. ?armelia
centrata
S. Lettcodon
.julaceus
IT.
0.
Same as M
Northeast
side
35.28
12.2 meters
37.06
59.60
25.43 C
21 C
17.55
t rone lo ides
2. Leucodon
.julaceus
4.9
from ground
south side
J. virginiana
P.
H.i::."
Same as
Horthwe t
side
V.
25.7 C
13.7 metevs 41.28
from ground
northeast
57.1
29.03
21.1 C
17.9
5.3
£.stellat^
l.Uo epiphytes
l. Parmelia
centrata
vmlf enii
41.52
58.43
39.76
46.79
12.2 m.
Northeast
J.virginiana
13.7 meters
l. Parmelia
2. Pertusaria
stel.lata
north-vest
l.Ramalina
calicaris
2. Parmelia
centrata
centrata
2.Ramalina
calicaris
58.05
Same as R
south side
T.
5»4
37.06
1.2 meters 38.52
from ground
south side
Q,. stellata
Q..
l. Leptogium
47.28
6.8
5.8
Same as for
station H
l. Parmelia
centrata
2. P.. calicaris
58.11
25.43
14.3 1. P. centrata
2.Epra chlorina
3.P.Mlfenii
42.
View that the evaporation
,
or to te more exact, the
evaporating power of the air is prohahly the most
important factor in the distribution of the
epiphQrte,'^
The total aver^ea for evaporation both on the juniper
and on the oak are shown in the graph (Fig, 3.3) and the
daily
averages
of evaporation, light intensity, air
temperature, substratum temperature, and the relative
humidity of the air for each height and exposure
on
the trtmk of the tree, with the corresponding epiphytic
flora, are tabmlated in Table II, These readings are
given for each period during which records were obtained.
In Table III are given thdtotal averages for all the
periods for each individual station on both the juniper
and the oak which are arranged in the order
la
of the
te of evaporation, beginning with the station that
has the least evaporation rate and ending with that
showing the maximum rate.The corresponding epiphytic
flora for each station and the total averQaes of the
other factors are also presented in the table
it is
possible
so thAt
to see at a glance the relation
of
the occurrence of the epiphyte to the climatic
conditions, It appears from the tabulated results
that where the evaporation rate is low, the epiphytic
fern is present and where the evaporation is
hi^
the
polypody is wanting. When these readings are presented
graphically the fact becomes even more obvious, as is
shown by the graph (Fig, 14), that the epiphytic flora
A
43.
of the tree has a definite regional distribution in
both vertical and radial directions and this vertical
and radial distribution is infuenced primarily by the
evaporating power of the air, Thus, v;here the evapration
rate is the lowest the epiphytic flora consists of
hepatics (Fr ul^ia virp:inica ),of various mosses, and
of Polypodium polypodioides» As the evaporation rate
increases the hepatic begins to disappear leaving only
the mosses and the fern;v;ith the further increase
of the evaporation the fern disappears and lichens come
where the evaporation is
in, In regions on the tree
still greater, the mosses disappear leaving the lichens
as the sole epiphytes, Evidently the evaporating power
of the air plays a very important part in the dictributioa
of the epiphytes on a tree, When each of the other
conditions is presented
graphically in the same manner
as the evaporation; ths,t is
starting with the lowest and
,
gradually going up to the maximum, then, when the
corresponding epiphytic flora
given, there appears to be
for each station is
no definite relation between
the distribution of any of the epiphytes and that of
the maximiHii effect of any one of the other
factors,
study of the graph of the light intensity (Fig, 15)
suggests that the hepatic occurs where the light is
the weakest, the mosses and the fern seem to endure
light of more or less moderate intenGity,v.hile
the
44.
lichene occur in weak light as
v/ell as in
strong
light. Apparently the light factor is not the most
important one, though it undoubtedly exerts some
influence on the evaporating power of the air and
in that way affects
tie
distribution of the
epiphytes indirectly. The curve of the air temperature
1
45.
is an indirect one, Its direct effect is really
on
the evaporating pov/er of the air rather than on the
distribution of the pla^s on the tree. The arrangement
of the epiphytes according to the relative humidity of
tha air (Fig, 17) also suggests an indirect effect on the
regional distribution of the epiphytes on the tree.
Here, it appears, that where the humidity is the greatest
the hepatics occur; where it is the lowest the lici^ns
prevail. Apparently, the effect of the relative humidity
of the air on the occurrence of the mosses ad the
polypody is not a very marked one since both of these
plants are found where the humidity is low as well as
where it is relatively high.Hovrever, neither the air
temperature
nor the relative humidity of the air
present any marked variation on the different stations
on the tree, The curve of each is practically a straight
line while the light
and the evaporation curves are
distinctly ascending curves indicating that
the intensity
of these was quite marked at each station on the tree.
The substratum temperature curve (Fig, 18) does not seem
to show any definite relation to the occurrence oflthe
epiphytes on the tree.
It is safe to conclude from the results obtsdned
that neither the light, the realtive humidity of the
air, the temperature of the air, nor the temperature of
the substratum exert any direct influence on the
d
TT
«*-
'V
ii
^
e:
I ^
.
^
^J
-
o. ?-
T-
_:
"^
J < J J.J J ^ J J
_5TATi0nS
STATIONS
raph shov/ing the
F±[^»/6
relation of the temperature
Graph showing the
influence of the relative,
humidity of the air on
the distribution of
epiphytes.
.1}-
of the air to the distrilDution
of the epiphytes,
L- Li Chens
H.-Hepatics
M- Mosses
P. Polypodiuin poly 'oodioides
STATlloHs
is
I
showing the inf lixence
--
L-Lichens
-.•
'
'
-:
''1
oi
'r :iture
of the epiphytes,
H-Hepatios
M-Mosses
P- P , p ly p di gj. es_
46.
occurrence and distribution of P. iDolypodioides
nor
on the epiphytes associated with it .It is the
sum total effect of the first three conditions
on the evaporating power of the air in
the immediate
vicinity of each station on the tree whi ch is
responsible for the definite
epiphytic flora on the
given station,'
B, CHEMICAL ANALYSES
Having determined
the influence of the climatic
conditions on the distribution of Polvpodiiim
polypodioides the writer
was confronted v/ith the
problem of the source of water supply and of mineral
nutrients needed by the epiphyte in its development
and growth, In order to determine whether the epiphyte
derives the necessary material from the substratum v/hich
it inhabits or from the dust of the air .chemical
analyses
were made of the "humus" gathered from the
bark of the tree on which the epiphyte was growing.
An analysis was also made of the epiphyte itself.
To determine hov/ much of a contribution to the chemical
composition of the "humus"the bark itself made, the
bark of the trees that bore epiphytes was analyzed
and likewise the bark of those trees that did not bear
them, In connection with this work, the writer wishes
TO
express his thanks to Dr,W*F,Hand, State Chemist
of
Mississippi and his
assistant Mr, H.Solomon for
•
y
»
*
*
47.
TABLE 17
Material
CHSIICAL ANALYSIS
Mcisture
Sangple
Ash
Si
N
K
Phosphoric Remarks
Acid t
f^
Garya alba
5.4
Ivofas
35,2 0.48
small
clTinip
at ground
Carva glabr a "
Carya ovata
Crataegus sp. "
4.35
8.30
13.30
32,5 0.67
20.2 0.70
4.2 1.52
20nin oluuip
on stem
DiOSPTTOS
Yir>?iniana
23.7 0.76
7.83
small cluinps
at ground
Jtntiperus
Ash of
virgjniana
htnrras
It
Bark
25.40
Hunnxs
p.3.30
Pimts taeda Bark
10.85
0.38
0.51
1.65 0.05
1.7*
1.9-
3.34
0.45
0.21' 0,10*0.15*
O.24^fp.ll#0 .17#
Ash of
bark
5.89
8.84
3.85
3.76
5.10
4.89
large clump
on stem
No epiphytes
PoLvTDOditnn
uolypodioide s Ash of
plant
Plant 9.33
*1.38 0.66*0.38
1.52#0.73#l0 .42#
7.46
8.23#
QuercTts falcbta Htnnqs 11.0
12,2
1.07
Epiphyte on
O. stellata
Epiphyte on
J.vitginiana
Epiphyte on
g. stellata
SOnni
cluB^
at ground
0« stellata
Hunus
10.5
15.6
17.5#
*1.7 *0. 08*0.27
0. Z:'- 0.09f0.30#
12.7*
13.941
0.51" 0.05» 0.11*
0.56# O.O&f 0.12#
0.391 0.87
HtJge cltmip
on trunk
Bark
8.85
Ash of
bark
Ash of
0.51
htnaus
1.73
TJlaus
apericana
Hxmnis
2.35
37.0
0.23
small cluop
at groxcnd
10.35
12.3
1.25
(*) Calcnlated on original basis
"
"
(#)
"dry
Large olxmsp
on trunk
48.
making the chemical analyses. The results of these
analyses
aire
presented in Talale
A study of the data reveals
IV,
the fact that wijere
nitrogen was abiindant, the clumps of the epiphytic fern
were
fairly large, That the nitrogen was not obtained from
the dust of the air is evident
which showed a high
showed
from the fact that samples
percentage of silicon, generally
a low percentage of nitrogen; where the. silicon
content was low the nitrogen content of the sample was
high, Regarding the silicon content as an index of the
amount of dust accumulated in the crevices of the bark
(since silicon is abundant in the dust of the region),
it appears that the main source of nitrogen
the dust
was not
of the air but the accumulated decaying
organic matter which composed the bulk of the humus
and which eradiated on the bark through the masses,
of epiphytic bryophytes and animal matter associated
with them,The luxuriant growth of therern on the
substratum rich in giitrogen indicates
content of the substratum has a marked
that the nitrogen
effect on the
development and grov/th of the polypody ,This is particularly
evident in ta case of Crataegus
which, in spite
possessing a smooth bark and being only slightly
of
over
eight centimeters in diameter, vas able to support a good
sized clump of the polypody, doubtless due to the
nitrogen content of the humus on which it thrived.
high
49.
Although the ash of the epiphyte, as a rule, shows
a high percentage of potash, the "humus"
genera,
lly
shows a low potash content^which seems to indicate
that most of the potash obtained by the plaint comes
from the dust of the air. This is probably true also
for the phosphoric acid, for very small sunounts of this
substance were found in the "humus", while the ash of
the epiphyte shov/ed a much higher
percentage of the
acid, It is possible that the high phosphoric acid
content in the ash of the epiphyte is due to metabolic
processes.lt is evident from the chemical study that
the fertility of the substratum is of as much importance
to development
and growth of the fern as are the low
rate of evaporation and an abundance of moisture
for
the beginning of the epiphyte on the tree, It is indeed
remarkable that the nitrogen of the humus on the bark
of the tree is frequently as high and at times even
higher than the nitrogen content of the most fertile
so]]iyS,for
very few soils have as much as 1,6% nitrogen.
It is no v/onder, then, that the polypody thrives so well
on such trees as Q.uercus stellata. Juniperus virginiana
.
or Ulmus emer i cana and developsion these trees an extensive
root system
euad
an abundance of leaves.
Acidity and Alkalinity Tests of the Substratum.
Chemical determinations in the field, by means of
the La Motte set of indicators, cf the acidity of the bark
of trees on v/hich the epiphytes occurred and also of the
50.
"baxk of those trees
which
no marked difference
Ijore no
epiphytes, showed
in the acid content, In fact,
the acidity of the "bark
ot*
of the "humus" on the
bark appeared to vary with the moisture content
Hs^
of the air as v;ell as
with the time of the
day, It was found that the acidity was slightly
early
in the morning than it was
a rain or immediately after,
the.
hi^er
at noon,ajnd during
bark or the "humus"
also showed a slightly higher acidity than in dry
weather, It was found further that the bark of those
trees which never bore
any epiphytes contained
aja
acidity of the same concentration as the bark of those
that bore
an abundance of the epiphytic fern and
mosses, The teste also revealed
tlat
the fern occurs
not only on slightly acid^soil" but is frequently
found also on slightly alkaline "soil", It is true,
however, that on the bark of the pine which exhibited
an acidity of 4,5 (P„
-/allies)
the fern never occurred.
There, it is believed, the real cause
of the absence
of the epiphytes was not the high acid content of the
bark, but the physical character of the pine bark
(
ginus
taeda and P,echinata),It thus appears from a study of the
chemicail reactions that acidity or alkalinity of the
substratum does not influence the occurrence
ribution of Polypodium polypodioides.
ajid
dist-
s
51.
C.
EFFECT
OP PHYSICAL CHARACTER OF THE BARK
ON THE DISTRIBUTION OP P.POLYPODIOIDES
6lDservations in the field indicated that the
fern occurred abundantly on trees the "bark of
which possessed crevices.lt was particularly
abundant on bark which was deeply furrowed and
which was sufficient3.y soft to
allov/ the
young
roots of the fern to penetrate into the interlaminar
spaces oOn such bark the polypody was found
developing a copious root system in the interlaminar
spaces, where the young root hairs were always found
to be turgid and functioning regardless of the weather
conditions, A flaky bark such as that of the juniper
Fig, 19
The underside of a mass of P« po lyppfli o i de
showing the massive root system in* the interlaminar
spaces.
52.
also appears to
"be
suitable
for this epiphyte. The
thio^mess of the bark, is, apparently, also important
since in thick bark the young roots can make their
way into the spaces between the layers fartherst".
away from the surface and be protected from excessive
evaporation. The polypody has never been found on trees
with smooth bark nor on those the bark of v/hich breaks
or peels off frequently,
as in the case of the pine
or sycamore,
A study of the water absorbing power and water
losing power of the bark of different trees seems to
indicate that only those barks v/hich absorb and hold
water
fast serve as
satisfactory substrata for these
epiphj'-tes.The bark of the pine is not suitable for
the development and growth of the epiphyte, for water
is absorbed rapidly ani
is lost just as rapidly so
that the bark becomes very dry
been wetted. Nor does
shortly after it has
a bark v/hich is horny or stony
in character, as that of the red oak or of the hickory
nake a desirable substratum, for such barks have a
TB
ry low imbibing pov/er. These conclusions were reached
after samples of bark of different trees were soaked
in v/ater for tvienty four hours suid then dried for
tv/enty four hours, their weight being detei^nined before
soaking, after soalcing and after drying.The 7/riter realizes
of course, that this methodjis not absolutely precise.
53.
for it Is quite possible that in some "barks the water
might never have reached into the interior of the
sample, while in others the water undoubtedly penetiated
far into the interior.But it was impossible to cT3cure
definite blocks of bark which could be measured in
three dimensions and their swelling or shrinking areas
determined, for that reason the gravimetric method
was used which although not very accurate presents,
nevertheless, an explanation sufficiently plausible and
which seems to coincide with the explanation derive!
from the field observations,"
D.DISGUSSION
From the study of the distribution of the
epiphytic polypody, it may be stated that the fern
begins its life on the tree in either of two
v/ays.
It may start from spores blown into the crevices
of the ba: k or on a mass of mosses on the
tree trunk.
These spores if they find sufficient moisture germinate
and develop prothallia which later produce
young
sporophytes.Such stages of development of the fern
were found abundantly on the trunks of Juniperus
virginiana both in the autumn and in the spring.
The other mode of origin is that from the vegetative
fragments of the cporophyte which are v/ashed down from
higher branches to the lower ones by heavy rains.
54.
This apparently happens on Quercus stellata where small
clumps of the epiphyte v/ere found in the crotch of
a lower "branch immediately below a huge mass of the fern
two meters above .A search for young sporophytes o r
prothallia proved unsuccessful.
It was found further
araericana and Juniperus
that Quercus stellata .Ulmus
virginiana
serve as the best
supports for the epiphyte, for on these the polypody
v^as
found in abundance.On the oak the epiphyte invariably
inhabits the oldest branches and the main trunk on the
north, northeast, and northwest sides, On the elm it is
found often inhabiting the trunk from the very ground
to the oldest branches, On the juniper the epiphyte
generally occurs in small clumps all along the stem.
It may be added, that on the latter tree the fern
was
never found in the fruiting stage, which seems to indicate
that the bark of this tree is favorable
of the epiphyte but that long before
the bark together with the
polypody
for the start
the fern matures
is shed, This was
foimd true in many cases, for a number of clumps of the
fern were found lying
on the ground apparently recently
detached from the tree.
This definite distribution of the
epiphytes in
relation to climatic conditions and to
55.
their supporting plant has interested botanists for
a long time. The early workers, in fact, attri^ted
the occurrence of epiphytes in some places and their
ahsence in others chiefly to differences in light and
Schimper p. 90 (25),
atmospheric humidity, To quote
"Licht, feuchte Luft.reichliche ThaulDildung,ha«fige
Regenguesse stellen die wessentlichen Bedingungen eines
ufeppigen
epiphytischen Pflanzenlebens dar,"It was
Byer (2), however,
who suggested that "only
can live as epiphtes.
..
such
plants
.which can stand the hot sun and
the drying winds," In other words, he suspected that the
water supply and water loss is the most important problem
of the epiphyte. His suggestion indicates that only those
plants can exist as epiphytes which are capable of
resisting excessive evaporation.
That the evaporating power of the air is actually
8J1
important factor in the occurrence and distribution
of the epiphytes is evident in the
The graphical
case of P.uolvioodioides
representation of the results obtained
(Fig, 14) as tabulated in Table III shows clearly that
it is the evaporating curve alone which bears a definite
relation with the distribution of the epiphytes. Thus,
where the evaporating power of
the
air is low the
hepatiOB, the mosses and the polypody occur; where the
evaporation
rate becomes high, the hepatics begin
to disappear, while the mosses and the fern still persist;
as the rate of evaporation
increases thejfern disappears
J
56.
"but
the lichens "begin to appear, With the further
rise of evaporstion, the mosses also disappear leartring
only the lichens to occupy the areas of highest evaporation.
The definite distribution of epiphytes on the
tree
corresponds somewhat to the results obtained by
Shreve (26) in the Blue mountains of Jamaica. He says
p,!5y#**^e trees of the upper slopes
epiphytic flora
have the same
that v/ould be found in the upper
two-thirds of the trees of the ravines, The trees of
the ridges and peaks have only those that are
characteristic of the uppermost third and the midheight
species are restricted in these habitats to the side
of prostrate trunks or fallen logsJThus, the vertical
and radial distribution of epiphytes on an individual
tree appears to be comparable to a certain degree t o
the similar distribution of epiphytes in the mountains
Not only idthe vertical
distribution of the epiphytes
on an individual tree well marked but even at the same
height different sides of the tree will bear different
types of epiphytes, as is shown in the ease of Q. stellata
where, for instance, at ten meters above ground the south
side of the trunk bears only mosses, while the northeast
and the northwest sides bear huge clumps of the poiypody
entangled in mosses, It was found, however, that at
different sides at the same height and at different heights
of the same tree the evaporating power of the air
i a
57.
not at all the same, which undoubtedly accounts for
the variation in the distribution
of the epiphytes.
The vertical distribution of the epiphytes on one tree
may thus be compared with the vertical distribution
of the vegetation on a mountain.The mosses axd
the
polypody correspond to the mesophytic vegetation of the
middle region of the mountain, while the lichens
of the dry regions of the tree are comparable
with
the xerophytic vegetation of the summit of the mountain
where the drying winds prevail .The hygrophyllous
vegetation which can not endure high evaporation, if
such is pre sent, would be found at the base
and would
correspond then to the hepatics in regions on the
tree where the evaporation is the lowest.
Not only do climatic conditions affect the dist«»
ribution
of the fern but even the chemical and physical
characters of the bark exert a marked influence on the
occurrence and development of the epiphytes, It appears
that a substratum rich in nitrogen causes a vigorous
growth of the epiphyte; the other elements seem to
be supplied in sufficient amounts
by the dust of the
air, Furthermore, the bark itself in order to make
a
desirable substratum for the epiphyte must be sufficiently
soft to allow the penetration of the young roots of the
fern, When that is done
,
the absorbing roots remain
practically imbedded within the bark,while those which
earlier fijinctioned for absorption are later found in
58.
in the outer layers of the bark and have retained only
the fiinction of anchoring roots which hold the fern in
place and also aid in the accumulation of humus, The
question arises whether this fern is to he regarded
as a true epiphyte when such conditions are established.
In answering this question
one may suggest a clas-
sification of epiphytes based upon that of Went though
somewhat modified.
Epiphytes may be classified into Eu-epiphytes,
Hemi- epiphytes and Pseudo- epiphytes. 'As eu^-piphytes, or
true epiphytes, we may regard those plants only which
no longer derive their minearl elements or water from
the substratum to which they are attached, These epiphytes
have undergone
some modification of their organs so
that they can obtain these substances from the air, The
attachment to the substratum is usually accomplished
by modified roots which function s%ely for anchoring
To this group belong many bromeliads and orchids,
Hemi epiphytes, or facultative epiphytes, are those
plants which although having lost connection with
the earth still derive water and other nutrient substances
from the substratum
to which they are attached by
more or less modified soillroots which are imbedded in
the substratum.These
epiphytes may occasionally occur
one
on the ground also, though tjieir most usual habit is
59,
on trees and dry rocks, Exaples of this type are
Polypodium polypodioides and P.vulgare
and many
other tropical ferns and orchids,' Pseudoepiphytes,
or false epiphytes, are those forms which are ordinarily
terrestrial plants
"but
by some agency or "accident"
they axe brought to the tree and succeed in growing
there for some time, These plants possess no special
adaptations and practically never reach maturity on
their supporting plant. Such epiphytes have no definite
regional distribution and are found in all parts of
the earth, As soon as condtions
for development and
growth become distictly different from those on the
ground, they wither away and die,'
60.
PirrSICLOGICAL STTJDIES
Tl?hile
studying the effect of the climatic conditions
on the distribution of the polypody, the writer obseirved
carefully
the peculiar curling of the leaves of this fern,
This characteristic "behavior of the leaves of
Polypodium
polypodioides in dry v/eather seems at first sight to serve
as a special, adaptation against dessication,-
Fig.20
The polypody in wet
weather with leaves
expanded
Fig. 21
The polypody in dry weather
with leaves curled up.
The curling on drying begins,
e.s
a rule, by the rolling
inward of the pinnae from the ends with the upper surface
inward,As the entire leaf is folding, each individual pinna
also folds with its upper surface inward so that there
is really a double folding in each pinna, one main rolling in
endwise of each pinna towards the rachis and the other a
61.
curling upv/ard and inward of the margins of each pinna towards
its midrib, The folding of the individual pinnae is soon
followed by the curling of each leaf and v/hile this takes
place the rachis begins to twist and to curl, Thus, in all cases,
the curling of the leaf causes the upper epidermis to be hidden
from the outside, while the lower epidermis, v;hich is covered
with numerous
whic/^
3CpJ.es, remains
exposed.This chsjcacteristic curling
conceals the upper surface and leaves only the lower surface,
exposed, suggests at once that this is an adaptation of the
leaf to xerophytic conditions.lt might be assumed from such
a behavior of the leaf that its upper epidermis loses
v/ater
more readily than its lower one, and by being rolled in when
the air is dry the leaf avoids the danger
of excessive
transpiration from the upper surf ace. The scales which cover
the lower surface of the leaf might well be assumed to function
as an aid in lessening
the evaporation from the lower epidermis.
To determine definitely whether these assumptions are correct
and to learn which side of the leaf really loses the greater
amount of v/ater
under given conditions, experiments were
carried on in the laboratory both on living and dead leaves of
the epiihytic fern, Abundant
material for these
experiments
was obtained from Mississippi through the kindness of Professor
J»M,Beal and a large clump of the living fern
vf&s
sent from
Athens, Ga, by Professor J,M,Reade to both of who the writer
here expresses his sincere thanks.
52,
Methods of Experimentation
Three experiments were performed on detached leaves
of P. polypodioides
»
In the first experiment four groups
of four leaves each were placed in a desiccator containing
a uniform layer of calcium chloride, In the first group
each leaf received a thin layer of petrolatum on hoth the
upper and lower epidermis thus sealing up both surfaces
completely. The leaves of the second group received a layer
of the petrolatum on the upper surface only
,
leaving the
lower one open; those of the third group were sealed only
on the
3i07/er
side, leaving the upper side
untouched; tut
the leaves of the fourth group were left in the natural
normally
condition so that transpiration could take place
from
"both the
upper
and lower surfaces .In each case only
the leaf blade was used, in order to avoid
arising from an uneven sealing of the
einy
complications
cylindrical petiole.
After the sealing of each leaf, it was 7/eighed and then dried
in the desiccator for twenty four hours and weighed
again
at the end of that time, The weighing was repeated dally for
one week.At the end of the experiment the recorded weights
of each group were averaged and the percentage of water lost
from each surface
was
Having thus determined
calculated from the results obtained.
the rate
of water loss
from each
surface of the leaves, these same leaves were then suspended
from wires in ahoist chamber so that
meter or so
ea.ch leai"
was
a centi-
above the v/ater level but not actually in contact
63.
with the water, This was done in order to determine the water
a"bsorbing power of
each surface of the leaf.As previously
done, the leaves were wei^^ed daily for one week. At the end
of this test the average percent of gain for each group
was then calculated.
In the second experiment the method was somewhat modified.
Instead of using calcium chloride as the drying agent, sulphuric
acid was used since hy means of this acid
it was possible
to establish different degrees of drying, Forty cubic centimeters
of concentrated sulphuric acid (lOO^) constituted the drying
agent in the first mason jar which was sealed tightly by
a pso'affined cork,From the inner side of the cork eight leaf
blades were suspended from hooks fastened in the cork,Of the
eight leaves, two were vasalined on both sides, two on the
upper side only, two on the lower side only, and two remained
uncoated for control.In no case were the leaves allowed to be
close to the acid; they v;ere generally about six centimeters
above the acid, The leaves in the second jar w^ere similarly
arranged but this contained
forty cubic centimeters of
diluted sulphuric acid in the proportion of one part
o i
water to three parts of acid (75^), A third jar contained
a like quantity of half water and
half acid (50^); a fourth
contained one part of acid to three parts of water {25%);
a fifth one part of acid to nine parts of water (10^); and
a sixth jar contained only distilled water (00^). Thus, each
64.
jar contained a series of leaves some with the upper surface
free;
some with lower surface uncoated; some with both upper
and lower surfaces clear and some v/ith both surfaces sealed
by the vaseline, It may be mentioned here that
whij^e vaseline
was used in preference to cocoa butter since the latter tends
to harden, crumbles and drops off when the leaf
begins
to
curl thus introducing an error when the leaves are weighed.
All leaves of each series being in the same jar were exposed
to precisely the same drying conditions .The very dame experiment
was also made
with leaves which had been previously killed
by being subjected to chloroform vapor for three hours, after
Y/hich time they became brown and began to curl, Thus, both
living
and dead leaves were subjected to precicely the same
drying conditions, Each leaf from every desiccator was weighed
once in twenty four
hours for one week.
The third experiment was designed to determine any
internal structrual chaiges in the leaf under either dry (curled!)
condition or the moist (expanded) condition, Small pieces from
curled and expajided leaves were cut from plants
gro;^-n
in the
open, These were immediately dropped into absolute alchohol
and kept there for thirty siK hours frequently changing
-the
alcohol to insure complete dehydration, The material was thus
fixed before
any internal changes could take place, Later the
ordinary method was used in imbedding and sectioning this
fixed
material, For fixing the sections on the slide no water
used,
v/as
nor was there any water or dilute alcohol used in the staining
65,
of the preparations, this being done in absolute alcohol
entirely, Throughout this proceedure the leaves reta,ined their
original shapes as cut in the field and the sections cut from
them shov/ed the difference between the internal structure of
leaf and that of the expanded one.
the curled
Results
The results of the first experiment show that normal
unsealed
leaves lost on an average 68^12% of their weight
during the first twenty four hours; during the second day the
loss amounted to 8,75^ of their weight;v/hile for the rest
of the T/eek these leaves lost no more in weight, The total
average loss from the
uncoated leaves for the entire week
amounted to 76,88^ of their original weight, The one externally
evident result of this enormous loss was the curling of the
leaf , The leaves of which the lower surfaces were sealed showed
aji
average loss during the first day of 11,16^, on the second
5.12^,on the third day 8,72^,on the fourth 0,32^ and on the
fifth l,07^,Ko more loss in weight took place during the rest
of the
v;^eek,
Those leaves
of which the upper surfaces were
sealed lost during the first day an average of 42,20^, during
the second 3,67^ and during the third day 7,4^,No further loss
was observed for the rest of the week. The leaves with both
surfaces sealed lost the least, the first day's loss being
6,04^,during the second day the loss was 1,33^ and on the fourth
day a loss
of
4.,
82% was observed,These leaves showed no further
loss for rest of the v/eek,Theoretically these leaves should
66,
lose
no water at all
"but
since a complete sealing
of the leaf is practically impossible, it may be assumed
that the recorded loss took place
from the unsealed edges
of the leaf .At any rate, the result of prime interest here is
that the leaves lost the most v/ater when both surfaces were
open, From the lov/er surface alone
53,18^
v/eek;
while from the upper surface alone the
a total
of 26,39^ during the v^hole week, It is
during the entire
leaf lost only
the leaves lost
evident from these results that the leaves
of this fern
lose
water almost tv/ice as rapidly from their lower surface than
from the upper one, This appears rather a puzzling result for it
is actually this side
in dry
that is exposed when the leaf curls up
weather, while the side losing the least water is concealc-eL
and subject to very little
The
desiceation
•
results obtained from the second experiment axe
indicated in Tables V and VI, A study of the data reveals the
fact that when the leaves were subjected to artificial drying
in the sulphuric acid desiccator tliey behaved essentially
as they did in the calcium chloride desicc^tor.In other words,
although the leaves were dried by two different methods, the
results were the same; in both cases the greatest amoxxnt of v/ater
was lost from the lower surface of the leaf , This is true not only
for living leaves, but likewise for leases which had been
killed by chlorof orm,It is safe to conclude from these
that the lower surface
results
although covered with scales and
apparently r3v":tpnt to drought is really the surface
which
67.
Table
Series
Y
Living leaves in stilphviric aciddesiccatort-ested
for their capacity to lose v/ater
Siirface
sealed
Percent loss in .veight dtiring
3d
5th
4th
1st day 2nd
6th
7th
Total
percent
loss
68.
loses water most rapidly#It is needless to say that when
an appreciable amoimt of water is lost,
a,
folding or a
curling of the leaf results. In both living and dead leaves
those of which neither surface was sealed, curled up
completely when subjected to dry conditions; those of which
only the lower side was sealed curled not quite as markedly
as in the former case; the leaves of which the upper side
only
v/as
sealed became only partially folded;while those
with both surfaces sealed remained completely expanded
throughout the experiment, The condition of the living leaves
remained unchanged in the jar containing 10^ acid and in
the one contanining distilled water indicating that there
was no loss of v/ater from the leaves .The killed leaves, however,
failed to show any loss in weight even in the jar containing
25^ acid.Furthermore, these soon became infected by a fungus
in the jars contaings 25^ and 10^ acid as w^ell as in the one
containing pure water.
The results of the experiments on the water losing
power of the leaves, which show that the lower surface
loses water about twice
as rap^'Jy as the upper ^discredit
at once the idea that the protection of the upper surface
by curling is a very effective
adaptation ac&inst excessive
trans pi rati on, The rolling inward of the upper surface does,
of course, reduce the transpiring
the more active
area
in half , but it leanres
half, which contains all of the stomata, still
exposed.The question, then, exises whether the lower surface
might not really be
especially capable of water absorption
69,
SO thp.t
a,t
once
v/hen a
"be
limited amount of v/ater is present it would
absorbed by this hairy surface.Dried curled leaves
were placed in a moist chamber in order to determine
the
water absorbing power
of each surface of
those unsealed leaves
which had been kept in the desiccator
of the leaf. When
and which were completely curled after drying,
placed in the moist chamber, they gained
v/ere
later
44,53^ of their
weigh-t
during the first tv/enty four hours and became fully expanded.
During the second day the gain was 4.84^, on the third 4,53^,
on the fifth 6,15% and during the sixth and seventh days the
leaves showed no more gain in weight. The average total percent
of gain for the v:hole week amounted to
whose upper surfaces only were sealed
60.10^,Those leaves
gained during the first
day 16.29^,on the second 3.45^,on the third 7.8^,during the
fourth 1,32'^, during the fifth 0,63^ and during the sixth
and seventh days together the gain
observed was only 0,62%
making a tota3]of 30. 61^. At the end of the fifth day these
leaves began to mnfolc.
r.nd
by the end of the sixth day they
were fully expanded, The leaves
whose lower surfaces were
sealed showed a gain of only 1.25^ during the
first day, of
1.64^ during the second day, of, 52^ during the fourth, and of
0.01^ during the fifth and sixth days together thus making
for the whole week a total
gain of only 3.42^.Thi£ gain in
weight was accompanied by no appreciable difference in the
i
physical appearance of the leaf. Whether any part of the observed
gain in these three experiments was due to the actual
condensation by the leaf itself of the water vapor present
70.
about it
is an unsettled question. It is certain that
no leaf actually came in contact with the liquid water
which covered the
floor of the chamber, It is quite possible
,
however, that during the night the water vapor was condensed
on the surface of the leaves so that the leaves may have
obtained
the water entirely by the imbibition
of the liquid
water condensed on their surf aces, and not by condensing
the water vapor that penetrated to the interior by
v/ay of
the
storaata#At any 3ate,it is noteworthy that the leaf increased
in weight nearly ten times as rapidly when the lower surface
was unsealed as when the upper one
was left uncoated. Whether
this enormous increase in weight is due to
the fact that all
of the storaata of the leaf are found on the under side of the
leaf and none on the upper side and thus'.dff er;Jiore absorbing
surface
is a question that needs further investigation.
The diiff^"">nce in the appearance of the leaf under
wet and dry conditions suggests that there may be
internal
structural differences between the expanded form
and the
curled one.Pairaff in sections were made of both curled
and expanded leaves fixed in absolute alcohol, These showed
a remau:kable difference in the tissues of the leaf»* The cells
of the expanded leaf seemed to be turgid and maintained
their maximum size and smooth outline, The cells of the curled
leaf, on the contrary, greatly reduced in size and wrinkled
in outline, The shape of a cross section of a curled leaf
is shown in Fig,22,The diagrammatic drawings in Figs. 23 and
24 show the comparative size and shape
of the cells from
71.
a cross section respectively of a curled leaf and of am
expanded one, Both of these sections come from the same
regioniof corresponding pinnae, and the same magnification
is used for Taoth,
Whether the loss and gain of water by the leaf is entirely
an osmotic phenomenon or is accomplished
only
"by
imbihition might be sinswered
partly or entirely
by a measurement of the
cells of the leaf in the curled condition and of those of
the leaf
in the expanded condition, Parai'fin sections of
curled leaves were mounted on slides, the paraffin dissolved
off by xylol and the preparation covered
v/ith
a cover slip.
In this manner, v/hile the sections were still in xylol under
the cover glass, individuals? cells picked at random in three
different regions of the upper and lower epidermis and in the
palisade tissue were measured
in width and length, The two
dimensions of a niunber of cells of each region of a given
tissue v/ere then averaged and ths se were regarded as the
dimensions of a typical cell of the curled leaf, After the
measurements were obtained
,
the xylol was drawn out from
beneath the cover slip and replaced by absolute alcohol
while the sections were still
under observation under
the microscope, When all the alcohol was drawn out water was
introduced at one edge of the cover glassiand its effect
on the cross section of the curled leaf noted. As soon as the
water came in contact with the cell wall of the cells in the
section, the cells began to swell and to expand so that at first
a vertical straightening of both endS/.^took place and shortly
72.
Table VII
'
Increase in size of cells
leaf after v/etting
in a cross section of a ctirled
73.
after, the midrib region expanded and in straightening
forced the two ends in opposite directions resulting thus
in the expanded shape of the normal leaf under moist
conditions.By this means
measure the cells
it was possible
to actually
in the curled state and then again
of the same leaf in the expanded state, When the sections
were fully expanded ,a number of cells
in each of the
three different tissue layers in the different
regions
of the leaf were again measured, precisely as in the case
of the curled section^The results obtained
the dimensions of the cells
from measuring
in both conditions are
tabulated in Table VII, It is evident from the data
all the cells of the leaf do not increase
thd/:.
in size equally
when water is introduced,Tlie cells of the upper epidermis
above the midrib increase about tv/ice as much in width as those
of the epidermis on the lower side of the midrib, Tlie increase
of the cells near the margin in the upper epidermis is slightly
less than in the lov/er epidermis; while the epidermal cells
in the region between the margin and the midrib of both the
upper
EJid
lower sides show a considerable increase, though the
cells of the upper
side
increased su^bout a third more than
those of the lower side, It will be noted that the greatest
increase takes place in the width of the cells of both of these
tissues; the increase in length id decidedly smaller and is
practically alike in both tissues, In the palisade tissue the
greatest increase in length and width occurs
in the region
betv/een the midrib and the ma,rgin, while the smallest increase
74.
occurs at the margin.
If
v/e
regard the increase in size
after the introduction of v/ater as
aji
of the cells
index of their capacity
to lose water during drying, it tecomes evident that the
cells of the upper
epidermis at the leaf margin must lose
least water, while those of the midrib region must lose most*
The lo\7er epidermal cells of the midrib region, however, lose
less water than the upper epidermal cells of the same region.
It is further evident that
the greatest decrease in size, under
dry conditions, in all the cells measured^is that in width.
It thus appears that the curling of the leaf is caused
by the unequal
ra,te of
chiefly
water^loss of the cells of the different
parts of the leaf.
Physiological Significance of the Scales
The presence of the scales
on the lower surface
of the polypody suggests its possible
function a>
a
protection against dessication, According to ICarsten (14)
the scales present
on the underside of the stem of Polyp odium
imbrioatam serve to distribute the water drops which settle
on the stem. In Poly podium sinuosum.h ov/ever. the scales ser"W«
for absorbing water which appears on the stem in the form of
drops,Schimper (25) and Mez (18) both attribute
the function
of water absorption in Tillandsia to the scales, Mez regards
the mechanism by which the water is absorbed by the scale as
either osmotic or capillary, So far as the writer knows the
function of the scales in P. polypodioides has not been detBBmined, Although in a number of sections of dry leaves the v/ing
c;Y
75.
of the scale was found pressing closely against the surface
of the leaf, other sections, on the contrary, seemed to show
that the scale stood up a"bove the surfao
was
.-^
-".hat
there
a space "between the surface of the leaf and the wing
of the scale, A similar beha.vior of the scales was also
observed in sections of the expanded
difficult to grascrihe
leaf, It is therefore
any definite function to the scales,*
There are some indications
that they
aid
of water for the thick walled cells of the
to absorb water
rapidly, Whether
thick cell walls of the wing
\jQ$k
in the absorption
wing were seen
the v/ater aborbed by the.
later carried over by
capillarity to the living stalk eells
euid
from there to the
interior of the leaf the writer is unable to say v/ith any
degree of def initeness.It is also possible
that they help
to protect the plant against drought, Their primary function
seems to be, however, that of holding a film of water on the
under surface of the leaf long enough to allow sufficient
water to be imbibed by the cell walls
and in that way to
help restore the leaf to the expanded form,This is particularly
suggestive
when one considers the short time it
t9,kes
a
dry and curled leaf to expand after a rain, Long before the
;
roots v/ithin the bark are able to take up and supply to
the leaves
the water absorbed
by the bark during a rain,
the 3«r.Ies by holding a film of v/ater over the entire surface
of the leaf allow the surface cells to absorb water and to begin
to enlarge
3010.
thus to begin the uncurling of the leaf,"
76.
Relation of Water Content of Leaf to Leaf-movement,
It appears obvious both from the physiological study of the
le^
euid
from the study of its internal gtrucure that
evaporation takes place more rapidly from the lov/er surface.
The peculiar curling of the leaf which exposes this
surface
pjrd
conceals the upper one, and the presence of an abimdance of
scales on the lower surface are both concerned in producin^f
the striking change that occurs in the tissues of the leaf
under dry conditions, When the leaf is placed in the dessicator
v/hile still expanded the scales stand out somewhat so that
there is
a space between the wing of the scale and the surface
of the leaf,When traoispiration begins the lov/er surface containing
the stomata naturally loses more
water
than
the upper one
which is devoid of stomata, A stage then is reached
when the
cells of the lowdr epidermis begin to shrink due to the loss
of water, At about this time the scales begin to be drawn
close to the surface due to: the shrinkage of the
loY.er
.:
cells of the
3pidermis,Such a shrinkage would naturally be expected
to cause a curling of the leaf v/ith the lo7/er epidermis on the
inside and the upper one on the outside, A curling of this sort
v.'ould
seem the
lower surface
leg,!
•'.''.
iitcome of the known facts^'since the
loses the greater amount of water, In reality
quite the contrary occurs, It is the upper epidermis that is
found inside in the curled leaf, while the lower one is alv/ays
on the outside .The explanation for this unexpected curling
77.
must
sought in the different changes in size of the cells
"be
of the two sides
"by
measurements
of the leaf
it dries, These are indicated
as
made of the cells
in curled auid expsmded
leaves.
Measurements of cells of the upper epidermis, of the
palisade
and lower epidermis from three different regions
of the leaf seem to show that the lower epidermal region
shrink least in width, while those of the same region in the
upper epidermis
shrink most in v/idth.On the other hand, the
shrinkage in length is practically the same
of both the upper
and lower epidermis near the midrib.
It is further evident that the shrinkage
cells in width corresponds more
of
in the cells
to that
of;
the palisade
of the cells
the upper epidermis than to those of the lov/er
epidermis.
Furthermore, the shrinkage in v;idth of the cells :^ of ::the
lower '.epidermis in the intermediate region is much greater
than in those
at the same epidermis
near the midrib, while
at the margin the shrinkage in width is practically the
in both the upper and lower epidermis, Correlating these
same
;
measurements with the loss of water and the curling of the
leaf the following
explanation may be offered
for the
characteristic curling of the leaf, While the cellc rf uh3
lower epidermis
v^ater,
are decreasing in size ov/ing to the loss of
their osmotic concentration is undoubtedly becoming
greater than that of the mesophyll cells; this causes
a withdrawl of the water
,
therefore
from the mesophyll cells by osmosiaw'
78.
When this takes place, the cells of the lower epidermis
again increase in size v;hile those
of the roesophyll
palisade
tissue decrease and ultimately draw water out of the
cells. But the palisade cells soon begin to decrease in size
ultimately
and they
upper epidermis
water
drav/
water
out of the cells
of the
causing them to decrease in size, As the
is continually "being drawn out of the cells of the
lower epidermis by evaporation, these keep on replacing
the loss with water Obtained by the process of osmosis
from the mesophyll cellslUnd the latter in turn continue
to replace
t-i-v.""/^
by water obtained
?res
from the palisade
cells which likewise continue to absorb water
from the
cells of the upper epidermis, This process continues
until
a certain limit is reached beyond which the leaf does not
give off any water .When that stage is reached the cells of
the upper epidei-rais
have shrunk the most, since there was
no reservoir from v/hich they could draw their supply of v/ater
to replace the loss, Those of the palisade tissue shrink
as ramch as those
about
of the upper epidermis, while those of the
lower epidermis having had a continual supply during the
entire
process of evaporation shrink considerably less than
those of the upper tissues so that the cells of the least
size appear' on the upper epidermis and those of the greatest
size are found in the lower epidermis. For this reason the
lower epidermis is exposed during curling,v/hile the upper
one is concealed, This explanation seems plausible when
in size
consider the increase
(as shown in Table VII
evaporation.
)
v;e
of the cells during expansion
as an index of the shrinkage during
79.
Harshberger (ll) who studied the curling of Rhododendron
maximum in v/hich it is the upper epidermis
lower one fii'
l-e
comes exposed during curling, claims that
the rolling raovejnent of the leaf
passage
instead of the
,
is due to the gradual
of the sap into the •'.rtercollular spaces or due
movement of the liquid from cell to cell
"to the
protoplasmic bridges so that
by-
means of
one part of the leaf becomes
highly turgescent and the other part more or less flaccid".
He also believes that low temperature
is responsible
for the
moving of the liquids toward the upper side of the petiole
and of the leaf ;while a higher tempeaature reverses the process
It is believed by Harshberger that turgidity is the main
factor in the mechanism
It
v.'as
ojf
.
theB6:-movements,
found from a study of cross sections of a curled
leaf that the cells
showed
considerable shrinkage and
deformation, According to Steinbrinck
{2*f)
from a plant tissue not only decreases
the loss of water
the size of the
cells of the tissue and deforms them but also
affects the
concentration of the cell sap v/hich during the giving off
of water
turgor
pulls the cell membrane along v/ith it, As the
becomes diminished the membrane weakens
cohesion of the concontrated
EUid
the
cell sap draws the cell membrane
along v/ith it to the center producing thus
a decrease in
volume of the tissue. This explanation seems applicable also
to the decrease in volume
and to deformation of the cells
of the leaf in the curled state.
80.
OoT^c-jning the physics of the curling and ezcpanding
of the leaf of this epiphji-tic polypody, the writer believes
that the curling io not
entirely an osmotic phenomenon.
The loss of water from the cell
causes a shrinkage
so-called
due
sind
walls themselres probably
a deformation of the cell wall, The
"resurection" of the leaves
is probably largely
to imbitionof water by the cell walls. Such seems clearly
true from the behavior of these walls as it
v/as
observed under
the microscope, in cross sections of fixed and imbedded leaves
where no osmotic action could occur, These cells, many of then
cut open 30 that turgor is impossible, expanded quickly to
their normal plump form upon the introduction of water, In the
living leaf , however, there is reason to believe that both
osmosis and imbibition aid in the expansion of the leaf
immediately after a rain.UndoubtedlJr, the process of expansion
then
begins with an imbibtion of water by the cell walls and
later the osmotic action carries
the v/ater from cell to cell
v/ithin the leaf thus aiding in the distribution of the water
imbibed
by the cell walls.
81.
VI.HISTOLOGIGAI.
STUDIES
A study of ths structure of the root and of the root
hairs of P. polypodioides show first of all no siyns of
mycorrhizal association, The stucture of the root as whole
seems to present the appearance of a typical fern root, though
the root hairs differ somewhat from the ordinary type, Instead
of being more or less uniform in thixjkness as root hairs
generally are, these are swollen or "bulbous at the base and
taper off to a more or less blunt point at the tip (Figs«25Jkui.d
It will be v/ell to describe the development and mature
structure of the scales of the leaf of this epiphytic polypody
since this knowledge should aid in
solving the problem of
their function, A sagittal section of a very young leaf of
a mature plant, or of a young sporophyte, reveals the fa6t that
each scale arises from an epidermal cell, The scale is, of course
initiated very close to the grov/ing point of the leaf. The
epidermal cell from which the scale arises first divides by
a transverse wall which thus forms two cells, an. inner cell
and outer one, The inner cell remains on the level with the
epidermal cells, v/hile the outer one buldges out and soon
divides by a second transverse v/all forming
again an inner
cell and an outer cell, The oi/ter of these two cells continues
to divide several times forming a plate of cells, while the
lower cell by several successive
transverse and one median
longitudinal division, gives rise to
The cells of the plate
the stalk of tie
scale.
grow out more rapidly towards the
growing point of the leaf so that
in the mature scale the
82.
plate or v/ing of the scalelis wider towards the tip of the
pinna. The mature scale
consists of a sunken funnel-shaped
body the lower end of which is sunken in the tissue of the
leaf and the upper plate-like flat portion resting on the
surface of the epidermis. The plate-like portion consists
of a peripheral, memhraneous, radially rihbedjparanchymatous
celled wing and a central cluster of cells, The cells of the
wing are thick walled and are devoid of protoplasm as are also
the cells of the central clustr except that the latter are
brown in color, The cells of the funnel-shaped body, the stalk
of the scale, unlike those of the plate
contain protoplasqi
8,s
are thin walled and
is indicated by the presence of large
nuclei. Several stages in the devel praent of the scale are
shown in Figs,2i)-3|,,
SUIJMARY
1, Poly podium
polypodioides may occur on any side of a tree
and an at any height where
the environmental conditions
allov/,
2, Of the climatic factors
influencing the distribution
of the polypody, the first in importance is the evaporating
power of the air, High evaporation entirely prevents
grov/th of the
the
fern,
3, Light also plays a part in controlling the distribution
of the epiphyte, Its influence is probably not direct but
83.
through modifying the evaporating power of the air, it
exerts
an indirect influence on the distribution of the
fern,
4, The relative humidity of the air and the air temperature
affect the distribution of the fern only as they affect
the evaporating power of the air,
epiphytes studied the lichens
5, Of all the
the highest
can v/ithstand
evaporation; the liverv.'ort !Frullania virp-inica
endures the least evaporation,
6, The
capacity of the bark for losing v/ater rapidly affects
the fern only in its influence
roots
7, The
on the evaporation from th-«
of the fern v/hich are imbedded in the bark,
fern secures most of its necessary nitrogen
from the
humus accumulated in the crevices of the bark, The other
mineral elements
are obtained
more largely from the
dust of the air,
8, Where the humus of the bark shov/s a high nitrogen content,
the fern
grows luxuriantly, Where the bark is poor in
nitrogen only a poor growth of the fern is developed,
9, The
fern occurs equally abundantly on barks v/hich show
an acid reaction and on those
alkaline
which
shov/
a neutral or
reaction,
10;a deeply furroT/ed,sof t bark, v:hich has
a high water
absorbing power and loses water slowly, furnishes the best
substratum for the epiphytic fern.
84.
11, The leaf under dry conditions loses the greater amount
of water
from its lower surface with the surprising result
that a curling of the leaf occurs, which leaves the lower
epidermis exposed and the- upper epidermis concealed on the
inside,
12, The curling of the leaf
loss of v;ater
is perhaps due to
the unequal
from the cells of the upper and lower!
lev,
,:.r
epidermis,
13, The curling of the living leaf is apparently the result
of osmotic phenomena; the expanding is entirely due to
to both osmosis and imbibition
imbibition in dead leaves and
in the living ones,
14, The function
of the scales is
J
":
probably to facilitate
an equal and gradual distribution of water
surface of the leaf after a rain and als
o to
thrbughout the
absorb
Ir-^*^
v/ater,
that is present on the surface of the leaf , and to pass it
to the internal tissues of the leaf.
15, The scales
originate from epidermal cells near the growing
point of each pinna,
16,1:10
raycorrhizal fungus
?/as
seen
in the roots of this
fern,
I
desire to express my indebtedness and gratitude
to Professor Duncan S.Jolinson for much helpful advice and
encouragement reql'qved
during
the progress of the 7/ork
and tc express my sincere thanks to Professor Burton E, Livingston
for
helpful suggestions and for the loan of atmometers to
carry on this work.
85.
BIBLIOGRAPHY
1, Beyer, R, Weitere BeolDachtungen von
auf Wei den,
Verhandl.Bot.Ver.Branden'b.
2........
"
Ueberpflanzen"
35_:
37-42, 1893.
Ergetnisse der "bisherigen Arbeiten bezuglich
der Ueberpflanzen ausserhalb der Tropen,
Verhandl. Bot. Ver. Brandenb. 37;105-129.1895.
3,Czapek,F.Beitrage zur Morphologie und Ehysiologie der
epiphytischen Orchideen Indiens.
Sitzungsber.Kais.Alcad.Wiss.Wien 118_: 1555-180
3 textfigs,1909,
4. Dixon, W. A. On the inorganic constituents of some epiphytic
ferns.
Jour. and Proc.of the Roy, So c. New S.W, 15;175185,1881.
5.Geisenheyner,L. Zur Epiphytischen Kopfweidenflora,
Verhandl. Bot. Ver, Brandenb. 56;57-61,1894.
6.Goebel,K. Ueber Epiphytische Fame und Muscineen,
Extr.d'Ann.d'Jard.d'Bot.d'Buitenzorg 2:16'^3|1887.
7.
Morphologische und Biologische Studien,
Ann, Jour. d 'Bot. d'Buitenzorg.7_: 1-70, 1888.
8
Pflanzenbiologische Shilderungen Heft 1.
Epiphyten.pp.147-236,9 PI., 1389.
Water and mineral content of an epiphytic fern.
Amer, Fern Jour.9.: 99-103, 1919,
9. Harper, R.M.
lD,Earris, J.A.On the osmotic concentration of the tissue fluids
of phanerogamic epiphytes.
Amer.Jour.Bot. 5: 490-506, 1918.
ll.Harshbereger, J.W. Thermo tropic movements of the leaves
of Rhododendron maximum L.
Proc. Akad. Nat. Sc.Fnil. 219-224 3 textf igs,1899,
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VITA
Louis Jerome Pessin was^in Kremenchug, Russia, on
May 19th, 1894, He carae to the United States in November 1908,
He received his early schooling in the Russian Gymnasiura and
completed his high school education in the city of Nev/ York,
He entered the University of Georgia in 1912 and graduated
with the degree of Bachelor of Science in 191S,In the fall of
the same year he "became Rufus J, Lackland Fellow in the Shaw
School of Botany of Washington University at the Missouri
Botanical Garden, There he majored for one year in Plant
Physiology and Plant Pathology, In the fall of 1917 he entered
the Johns Hopkins University majoring in Botany with a first
subordinate in Plant Physiology and a second subordinate in
Zoology, During 1913-19 he served in the Medical Detachment of
the 146th Infantry in France and Belgium, He returned to the
Johns Hopkins University in April 1919 and in the summer of the
same year he studied the flora , especially the epiphytic flora,
of the Blue Mountains in Jamaica, B,V/, I, He v/as student-assista.nt
in Botany at the Johns Hopkins University during 1917-18 and
1919-20, He was instructor and later Assistant Professor of
Botany at the Mississippi Agricultural and Mechanical College
from 1920-/to 1922, He returned to the Johns Hopkins University
in the fall of 1922, As a result of his studies in Jamaica he
published a paper entitled "Epiphyllous plants of certain
regions in Jamaica" in the Bull, Tot. Bo t, Club 49_: 1-14, 1922.
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